Merriam-Webster's Collegiate® Dictionary, 11th Edition
SAP
I. noun
Etymology: Middle English, from Old English sæp; akin to Old High German saf sap Date: before 12th century
1. the fluid part of a plant, b. a body fluid (as blood) essential to life, health, or vigor, bodily health and vigor, a foolish gullible person, blackjack , bludgeon , sapless adjective saplessness noun
II. transitive verb (sapped; sapping)
Date: 1725
to drain or deprive of sap, to knock out with a sap,
III. verb (sapped; sapping)
Etymology: Middle French sapper, from Old Italian zappare, from zappa hoe
Date: 1598
intransitive verb to proceed by digging a sap, transitive verb to subvert by digging or eroding the substratum or foundation ; undermine ,
2. to gradually diminish the supply or intensity of , to weaken or exhaust the energy or vitality of , to operate against or pierce by a sap, see: weaken
IV. noun
Etymology: French sape, from saper
Date: 1642
the extension of a trench to a point beneath an enemy's fortifications